Switch runc and containerd startup to be entirely Go

At present they use a small shared function called "prepare"
that does the read-write remounts, that I will switch to doing overlay
mounts soon.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

pkg/runc/cmd/onboot/main.go

@@ -0,0 +1,63 @@
+package main
+
+import (
+	"io/ioutil"
+	"log"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+)
+
+const (
+	runcBinary   = "/usr/bin/runc"
+	onbootPath   = "/containers/onboot"
+	shutdownPath = "/containers/onshutdown"
+)
+
+func main() {
+	// try to work out how we are being called
+	command := os.Args[0]
+	if len(os.Args) > 1 {
+		command = os.Args[1]
+	}
+	var path = onbootPath
+	switch {
+	case strings.Contains(command, "boot"):
+		path = onbootPath
+	case strings.Contains(command, "shutdown"):
+		path = shutdownPath
+	}
+
+	// do nothing if the path does not exist
+	if _, err := os.Stat(path); err != nil && os.IsNotExist(err) {
+		os.Exit(0)
+	}
+
+	// get files; note ReadDir already sorts them
+	files, err := ioutil.ReadDir(path)
+	if err != nil {
+		log.Fatalf("Cannot read files in %s: %v", path, err)
+	}
+
+	status := 0
+
+	for _, file := range files {
+		name := file.Name()
+		fullPath := filepath.Join(path, name)
+		if err := prepare(fullPath); err != nil {
+			log.Printf("Error preparing %s: %v", name, err)
+			status = 1
+			continue
+		}
+		cmd := exec.Command(runcBinary, "run", "--bundle", fullPath, name)
+		cmd.Stdout = os.Stdout
+		cmd.Stderr = os.Stderr
+		if err := cmd.Run(); err != nil {
+			log.Printf("Error running %s: %v", name, err)
+			status = 1
+		}
+	}
+
+	os.Exit(status)
+}

pkg/runc/cmd/onboot/prepare.go

@@ -0,0 +1,21 @@
+package main
+
+// Please note this file is shared between pkg/runc and pkg/containerd
+// Update it in both places if you make changes
+
+import (
+	"path/filepath"
+	"syscall"
+)
+
+func prepare(path string) error {
+	rootfs := filepath.Join(path, "rootfs")
+	if err := syscall.Mount(rootfs, rootfs, "", syscall.MS_BIND, ""); err != nil {
+		return err
+	}
+	// remount rw
+	if err := syscall.Mount("", rootfs, "", syscall.MS_REMOUNT, ""); err != nil {
+		return err
+	}
+	return nil
+}